This invention relates to systems for fluids separation embodying a rotor within a housing and particularly to medical disposable units in which the rotor is retained by bearings and driven by an external magnetic drive. A particular area of usefulness of the present invention is hemapheresis, relating to the separation of one or more constituents of blood, using disposable filtration or centrifugal separation devices. A specific area of application is in a disposable plasmapheresis disposable device for the filtration of plasma from whole blood.
Plasmapheresis apparatus presently employed includes membrane filtration devices as exemplified by the device disclosed and claimed in application Ser. No. 591,925, filed Mar. 21, 1984 for "Method And Apparatus For Separation Of Matter From Suspension," by Donald W. Schoendorfer. Such membrane filtration device has an interior spinner which is covered with a filter membrane and includes a conduit system for collecting the plasma passing through the membrane. For plasmapheresis, such a filtration unit receives whole blood in the space between the spinner and the outer wall of the housing. The plasma in the blood filters through the membrane into corrugations on the surface of the spinner and then passes through radial holes into a central conduit. From the central conduit the plasma flows to the bottom of the spinner where it passes out through an outlet conduit in a pivot pin concentric with a central axis of the spinner device.
The spinner is mounted in a closed shell or housing and is driven remotely by magnetic intercoupling between a permanent magnet at the top of the spinner and an external rotating magnetic drive. The spinner is rotated at a high rate, such as 3600 RPM, to establish a precisely controlled enhanced vortex action which provides excellent plasma throughput rates without hemolysis. In the plasmapheresis application the disposable spinner is run, continuously or intermittently, for a period seldom greater than 40 minutes.
The upper end of the spinner is held in a pivot pin bearing during rotation and the lower end rests on the second, lower pivot pin bearing which has a central bore to permit downward passage of plasma filtrate to the outlet. An O-ring seal is generally positioned between the spinner and the lower pivot pin bearing to prevent mixture of whole blood or packed cells outside the spinner with plasma moving along the central axis.
In the plasmapheresis instrument presently employed there are four permanent magnets in the external drive mechanism. Such magnets are composed of samarium cobalt and are relatively powerful for permanent magnets. Barium ferrite magnetic elements, supplied in an unmagnetized state, are used for the rotor of the separator. After being magnetized with an impulse magnetizer, the resulting permanently magnetized elements are then cemented to the inside of a rotor cap which is then solvent bonded to the rotor. While this system is satisfactory in operative respects, it is relatively costly. It is essential that hemapheresis units of the type described above be manufactured as inexpensively as possible, because disposable units are used to prevent cross-contamination.
Moreover, the magnetic forces exerted by the permanent magnets are quite variable because the formulations of the raw magnetic materials are not precisely controllable. When the formulation varies, the magnetic field strength produced by the magnetization process varies, which in turn alters both the seal force and the torque available to rotate the rotor. Thus, the material of the permanent magnet and the magnetization process must be monitored very carefully to assure operation within narrow tolerances for operating consistency, and added cost factors are introduced.
The magnetic coupling between the magnetic structure on the rotor with the external permanent magnets on the drive mechanism must provide two functions. One is to generate sufficient torque to spin the rotor without slippage at high speed to provide superior filtration, while the other is to apply sufficient downward force to maintain the O-ring seal integrity without excessive wear.